Compounding rubber



Patented Feb. 10, 1942 2,212,000. COMPOUNDING RUBBER Frank W. Corkery,Grafton, Pa., assignor to Pennsylvania Industrial Chemical Corporation,a corporation of Pennsylvania Serial No. 2875623 No Drawing. ApplicationJuly 31, 1939,

6 Claims. This invention relates to the compounding of v(Cl. 260-4)rubber; and the application for Letters Patent of the United Statesthereon is in part a continuation of application Serial No. 130,907,filed March 15, 1937.

My invention involves the compounding of rubber with use of a residualgum of specialized sort. Such residual gum is produced by oxidation ofthe still residue obtained in the treatment of light oil from theby-product distillation of coal. The light oil comes over in coaldistillation, and is recovered from the gases with which it comes overby being absorbed in scrubbing oil which is ordinarily a heavy cut ofpetroleum.

The still residue with which I deal-results from fractionation andpurifying treatment of this light oil, following its removal from thescrubbing oil. This still residue contains polymers of resinformingbodies in various stages of polymerization, some heavy monomers, tracesof naphthalen'e, and various percentages of sulphates and sulphonates.It is a dark viscous oily substance deficient in drying qualities, anditself possessinglittle utility in the arts. As it is commerciallyavailable, the still residue may be obtained from either of thefollowing .operations conducted upon the light 011.

One such source of still residue is the still in which the light oil ispurified and fractionated to give crude benzol, crude toluol, No.1 crudesolvent naphtha (crude xylol), and No. 2 crude solvent naphtha.tlonation consists largely of 'heat polymers, and

.is known in the trade as "crude still residue.

The fractions from the crude still, after washing with sulphuric acidand neutralization, are distilled in what are known in the trade as purestills." The residues from the distillation of these fractions compriseprimarily acid polymers and heat polymers, and comprise alsosulphonates. The residues from the pure' still contain also sulphatesfrom acid purification and neutralization which have not been whollyremoved by washing. This pure still residue normally is mixed in a tankwith the crude still residue. so that, as it is commercially available,still residue contains at least the residual products from these twotypes of still dealing with coke-oven light oil and its fractions.

Usually-there is also mlxed'in with these two still residues in theresidue tank a third residue, which comprises bottoms from thedistillation of the heavier cuts of No. 1 and No. 2 crude sol-"above-noted co-pending application, I have discovered that these stillresidues or mixture of still residues possess to a substantial extentthe quality ofunsaturation, and that by taking advantage of thisunsaturated condition of the material it may be oxidized to produce anelastic and pliable material. which material I have utilized in thecompounding of rubber.

To describe the oxidizing treatment by which I I convert the liquid oilystill residue into an elastic and pliable material, I introduce thestill coils for circulating steam or other heating medium, and providedwith perforated coils connected'with a blower for introducing air underof the material and facilitate the dispersion of air through it, and anoxygen-containing gas,

' preferably air, is then introduced in finely divided streams, adjacentthe bottom" of the vessel, to pass upwardly through the body of stillresidue.

In so doing, the air is introduced under such pressure that it iscapable in dispersed condition of penetrating the body of liquid.

According to the viscosity to which the resultant elastic product ofoxidation is brought, the

The residuum from this fracintroduction of air is continued for agreater or lesser period of time. Thus by continuing the introduction ofair for a period of two days, while maintaining the still residue at theapproximate temperature of 200 F., a noticeable thickening of the stillresidue indicating the progress of oxidation is effected. A- treatmentof such duration, the supply of air in good volume being maintained,raises the viscosity of the still residue from approximately 200 to 250Saybolt at 212 1''. to a viscosity of approximately 600 Saybolt at 212F. In continuing the oxidation of the still residue, the temperatureunder which oxidation is further conducted desirably may be increased tofrom 200 F. to approximately 220 F., in order to facilitate thedispersion of air through the still residue undergoing treatment, andthereby. to expedite oxidation of the still residue.

At the end of an additional two-day treating period under suchconditions a still residue approaches the state of a solidat normal roomtemperature. Carrying oxidation further under temperatures ranging'up to245 F. or 250 F., the

still residue more rapidly increases in viscosity, a total treatingperiod of fourand one-half to five days serving to raise themelting-point of the still residue to from F. to F. The

. oxidizing treatment can be terminated at any point, or maybecontinued, if desired, to a-point at which the residual gum produced byoxidation has acquired a. melting-point as high as approximately 225 'F.It is the initial stages of the oxidation treatment which are prolonged;in-

creased viscosity and melting-point proceedingaerated rapidly as thestate of a solid has been approximated.

During the initial stages of the oxidation treatstill residue, andthereby to produce a residual gum of high elasticity.

The residual gum, oxidized by passing air, or other oxygen-containinggas therethrough, has been oxidized by what is known in the paint andresin industries as straight" oxidation. This is in distinction fromoxidation by means of chemical oxidants, in the use of which there issome decomposition involved in the progress of oxidation. The oxidizedgum difiers substantially from the unoxidized still residue, in that ithas acquired body and elasticity, and is oidecreased iodine value andsolubility. If carried to a melting-point approximating 80 F., itbecomes insoluble in petroleum distillates in which raw still residue isas to its preponderant content soluble. For my present purpose, namely.the compounding of rubber, this insolubility of the residual gum is amatter of primary advantage. The lowering of iodine value is 'a matterof importance for the reason that as incorporated in a rubbercomposition the oxidized still residue is not readily susceptible tofurther oxidation under the conditions in which the rubber is used.

I have found that I may, in compounding rubber, utilize the residualg'um within a relatively wide melting range. Also, I have found that inmilling it breaks down the rubber particularly well, thus facilitatingand expediting compounding of the rubber with other standard ingredientsof a rubber compound. The operation of compounding rubber consistsprimarily in milling the rubber in unvulcanized condition with fillingand toughening materials, of which common examples are carbon black,zinc oxide, and imusorial earth. Sulphur is also milled in, and inmodern practice some one or more accelerators of vulcanization, some oneor more anti-oxidants, and some one or more softeners and dispersantsare also used. Pine tar may be considered to be the standard softenerand dispersant used in compounding rubber. After the various additivesubstances have been milled into the rubber, the compound is vulcanized,or "cured" with heat, as in heated molds, in a steam-heated press, underhot water, or in some other suitable manner.

I may give an otherwise standard formula for automobile tire tread stockas follows. the parts being by weight:

Utilizing this formula, which may be considered standard, save for theuseof the residual gum as a softener, I have used chiefly residual gumof three widely difl'erent viscosities. One

example of the residual gum consisted of the still residue oxidized to amelting-point of about 90 F. Another example was this gum having amelting-point of about 90 F. in solution in an oily 'material formedchiefly of the lower polymers of indene, coumarone, and the like, in aproportion of about of the gum and 25% of the polymer oil. Anotherexemplary sort of residual gum which I have used has a meltingpoint ofabout 135" F. Different samples of the rubber blend, including thesethree sorts of residual gum, were subjected to cure at 280 F., each fortest periods of 30 minutes, 45 minutes, 60 minutes, minutes, andminutes. of these curing periods the one for 30 minutes is in the rubberindustry considered to be an under-cure, while a curing period of 120minutes is in the rubber industry considered to be an overcure.

In comparison with the effect produced in identical curing periods uponblends which were identical with the one given, save that they containedthe typical rubber softener, pine tar, in place of the residualgum, theresults are remarkably in favor of the residual gum. I have found thatthe tests on modulus of 300% and 500% show that a softer stock isproduced by use of the residual gum than is produced by use of pine tar.As reflected in the latter tests, it has been found that the rubbercompound containing the residual gum is superior to the similar compoundcontaining pine tar in its resistance to cracking. Resistance toabrasion is for similar curing periods in favor of the rubber compoundcontaining the pine tar.

Being of lesser stiffness than the rubber compound wntaining pine tar,the compounds containing residual gum are more flexible and show a muchhigher ultimate elongation. For all periods of cure the tensile strengthof the rubber compound compares favorably to a. similar rubber compoundcontaining the pine tar. It is higher when the compounds are subjectedto over-cure, an over-cure of the rubber compound containing theresidual gum showing approximately the same tensile strength as asimilar sample subjected to what may be considered an optimum period ofcure, while over-cure detracts greatly from the tensile strength of arubber compound containing pine tar. Thus taking as an example a sampleof rubber compound as above composed, which contained as a softenerresidual gum having a melting-point of about F., when subjected tocuring for 90 minutes at 280 the tensile strength was 4440 as an averagetaken from a number of test samples. When subjected to over-curing bycuring for 120 minutes the tensile strength was 4570. On the contrary,when subjected to a curing period 01' 90 minutes at 280 F. the averagetensile strength of samples containing pine tar was 4590. When subjectedto over-curing by curing for 120 minutes the tensile dropped to 4270.

On subjection to artificial aging it was found that the rubber compoundscontaining residual gum of varied viscosity showed no greater loss intensile strength and ultimate elongation than samples containing pinetar, when compared samples of the compounds were subjected-to anunder-cure. When subjected to an over-cure, loss in tensile strength andelongation was less for samples of compounds containing the residual gumthan for samples containing pine tar. Aging tests were conducted bysubjection to the oxygen bomb for periods of 72 hours.

" taining the residual gum show that the rubber compound containing pinetar is stiiler thanvthe rubber compound containing the residual gum.Both as to normal elongation and elongation after aging, the compoundscontaining the residual gum have higher ultimate elongation with respectboth to normal and with respect to aged samples than the compoundscontaining the pine tar. Comparing both the normal tensile strength andthe tensile strength ofaged samples for the three types of residual gumwhich have been taken as exemplary with pine tar. I have found that incomparison with pine tar the residual gum has no adverse effect ontensile strength but seems on the contrary to improve the tensilestrength of the rubber compound in which that the rubber may besubjected to contact'with it is incorporated. The tensile strength beingsubstantially equal for lower. cures and noticeably in favor of thecompounds containing the residual gum when subjected to over-cure, anoutstanding advantage of a rubber compound containing the residual gumis thus seen to be in the fact that it minimizes disadvantageous effectsresulting from over-curing the rubber. It is also a fact that theresidual gum does not act in part as an accelerator in curing thecompound, and its use therefore does not lead to what are in effectover-cures, but obtained accidentally in normal treating periods. It isa typical quality of the residual gum that it is approx-- imatelyneutral, having an acid value of less than 1. The various pitches, suchas stearin pitch, cottonseed pitch, pine tar pitch, and the like, whichcontain vegetable or fatty acids, tend initially to activate the cure,thus frequently causing unexpected over-cures. This is also true ofcreosote oil, due to the presence of phenols and phenolic acids.

The residual gum possessing in itself flexibilipetroleum oils.

It should be clearly understood that the formula for tire tread stock isexemplary only, since the residual gum may be used in various rubbercompounds as well as in varied proportions of the residual gum to theother content of the rubber compound. For example, it may also be usedin mechanical rubber goods, and can be used in considerable quantity,with particularly great advantage, for those purposes in which reclaimed.rubber and a large proportion of filler are used, such as compounds forrubber shoe heels, rubber mats, rubber gaskets, and the like. In all themany rubber compounds in'which it may be included, it performs the samefunctions which have been noted for it in the exemplary formula for tiretread stock given in detail above. For -'some purposes, the residualgum, may be compounded with the rubber in any quantity short of suchproportion that the compound ceases to be characteristically a rubbercompo Whereas the residual gum included in the examples given above hasin each example been v a gum solid at normal room temperature, it is ty,and in degree elasticity, may be incorporated in a rubber compound inlarger quantities than resins of various sort. Thus I have included inan otherwise standard typical rubber formula, such as that given above,from 15to 20 parts of residual gum, constituting more than 10%"of thetotal composition. Such. rubber compound I have found very susceptibleto abrasion, particularly for the lower curing periods. When over-cured,as, for example, when cured fora period of 120 minutes at 280 F., thecompound had substantially increased resistance to abrasion, and showedup well in comparison with wholly standard formulae with respect to itsultimate elongation and tensile strength, Such compounds I have found tobe particularly soft and particularly resistant to cracking. Theeflective aging was proportionately less than is the case of a standardrubber compounding formula.

' The desirable efiects of my residual gum, as

incorporated with rubber, may be attributed oxidation. Also theelasticity of the material gives it, in itself, rubber likecharacteristics, so that to a great extent it may act, when incorportedwith rubber, as a rubber substitute as well as performing the functionof a dispersing and softening agent in the rubber. The fact that theresidual gum is insoluble in' petroleum, acquiring such insolubility byoxidation, renders it suitable for use in rubber which is to be used forpurposes. such as vehicle tires, in which there is likelihood to beunderstood that for most purposes, including a rubber compound for tiretread stock, the residual gum may be of far lower viscosity. There is,however, a lower viscosity limit, a viscosity lower than which indicatesthat the initial still residue'has not been oxidized to a degreesufllciently to reduce its unsaturation, and which is of itselfinadequate to cause the gum to impart a to a rubber compound containingit novel and desirable characteristics. commensurate with those whichhave been noted. I have found that a viscosity of 1500 'Saybolt at 210F. is approximately the lowest viscosity at which the effects ofoxidation in thestili residue result substantially in advantageouscharacteristics of the rubber compound in which the oxidized stillresidue is included. v

Desirably, though not necessarily, the still residue is purified beforeits oxidation by removal 01' sulphatesand sulphonates of coumarone and Iindene, the presence of'which in the still residue has been above noted.This purification may be effected-in any desired manner, and may, forexample, be efiected by agitating the still residue with a low-boilingaliphatic hydrocarbon solvent in accordance with myUnited States PatentNo. 2,135,427, dated November 1, 1938. Whether purified or unpurified,its peculiar value in rubber largely to the fact that it is so oxidizedthat itsiodine value is substantially lowered by precompounding risesfrom its preoxidized condition. Since the still residue contains dimersand other polymers of coumarone and indene,- the dimers being thepolymers particularly susceptible .to oxidation, the purified andoxidized still residue is' in effect the oxidation product of a'low-polymer coumarone-indene resin; and my invention contemplates theuse incompounding rubber of such resin in oxidized condition. It is,however, a requisite of the oxidized resinous material which I use incompounding rubber, whether it be in the form of unpurified stillresidue, or in the form of low polymer coumarone-indene'resin, thatit bea gummy material resultant from a straight oxidation resulting fromblowing with air or other oxygen-containing gas, and that itshould notbe the product of decomposition and rearrangement resultant fromtreatment with oxidant reagents.

I claim as my invention:

1. A composition rubber compounded with a substantial proportion oflow-polymer coumarone-indene resin initially fluid at normal roomtemperature oxidized by blowing it with an oxygen-containing gas to thestate of a gum solid at normal room temperature.

2. In the process of compounding rubber the herein described step ofexpediting and improving dispersion of filling material through therubber by milling with the rubber and filling material a substantialproportion of a gum solid at normal room temperature resultant from theoxidation by blowing with an oxygen-containing gas of low-polymercoumarone-indene resin initially fluid at normal room temperature, as adispersing agent for the rubber and filling material.

3. A composition rubber compounded with a substantial proportion ofunsaturated aromatic still residue from the distillation of light oilderived in the by-product coking of coal, said still residue being aninitially oily liciuid oxidized by .blowing with an oxygen-containinggas to a viscosity of at least 1500 Saybolt seconds at 210 F.

4. A composition rubber compounded with a substantial proportion of aresidual gum solid at normal room temperature resultant from oxidizingby blowing with an oxygen-containing gas an an oxygen-containing gas toa viscosity of at least 1500 Saybolt seconds at 210 F., as a dispersingagent for the rubber and filling material.

6. In the process of compounding rubber the herein described step ofexpediting and improvingdispersion of filling material through therubber by milling with the rubber and filling material a substantialproportion of a residual gum solid at normal room temperature resultantfrom oxidizing by blowing with an oxygen-containing gas an initiallyliquid oily aromatic unsaturated still residue from the distillation oflight oil derived in the by-product coking 01' coal, as a disparsingagent for the rubber and filling material.

FRANK W. CORKERY.

